The present invention relates in general to current sensing for controlling inverter-driven electric machines, and, more specifically, to increased fault tolerance for current sensing in connection with electric vehicle drives.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main linking capacitor. A first inverter is connected between the main bus and a traction motor to propel the vehicle. A second inverter may be connected between the main bus and a generator to regenerate energy during braking to recharge the battery through the VVC. As used herein, electric machine refers to either the motor or generator.
The inverters include transistor switches (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor, or to invert an AC voltage from the generator to a DC voltage on the bus. In each case, the inverters are controlled in response to various sensed conditions including the rotational position of the electric machine and the instantaneous current flowing in each phase winding of the machine.
Pulse-width modulated (PWM) switching of the inverter is typically performed under control of a motor/generator control unit (MGCU) based on comparing a desired motor current with a measured current. For example, a measured current may be used by a torque calculator to calculate an instantaneous motor torque. A desired torque may be obtained from a driver input device, such as an accelerator pedal, and an engine control unit. A particular acceleration or deceleration indicated by the driver's action is used to determine how much torque should be delivered to the vehicle wheels. A difference between the calculated torque demand and the actual torque (calculated from the measured current) is used to update the duty cycle and/or operating frequency being used for switching the inverter.
Reliable current sensing is essential for proper functioning of the PWM control for the electric machines in an electric vehicle. The use of redundant current sensors can increase reliability, but use of multiple sensors for each phase has introduced calibration/consistency issues when using redundant sensors to take multiple measurements of one phase current. Furthermore, it would be desirable to obtain a greater increase in robustness beyond that achieved by known circuit topologies that have introduced redundant sensors.